1. Field of the Invention
The present invention relates to a telephone switching apparatus having a plurality of outside lines and extensions.
2. Description of the Related Art
FIG. 6 illustrates a telephone switching apparatus having a telephone line 50 and extension telephones 51 and 52. As is generally the case with a telephone switching apparatus, the telephone line 50 and extension telephones 51 and 52 of this system are connected through a private branch exchange 53, which performs channel setting. The private branch exchange 53 includes a time-division switch 54, an outside-line interface section 55, an extension interface section 56, and a switching control section 57. A predetermined channel setting is performed by the time-division switch 54, which is controlled by the switching control section 57.
In a telephone switching apparatus of the above-described type, telephonic communication with the terminal on the other end of the line may be conducted through a microphone and speaker. In that case, an extension telephone having a built-in hands-free circuit is employed for the purpose of preventing howling. The hands-free circuit is a circuit which compares the level of a transmission signal from the extension telephone with the level of a reception signal from the outside line, muting the signal at the lower level. The circuit is also referred to as an echo suppressor circuit.
FIG. 7 is a circuit diagram illustrating such a hands-free circuit. A transmission voice signal from a microphone 58 of the extension telephone 51 or 52 is amplified by amplifiers 59 and 60 and supplied to a comparator 62 through a low-pass circuit 61. A reception voice signal from the side of the private branch exchange 53 is converted to an analog signal by a D/A converter 63 and supplied to the comparator 62 through a low-pass circuit 64. The levels of the transmission and reception signals are compared with each other by the comparator 62. When the level of the transmission signal is higher than that of the reception signal, the output of the comparator 62 is "High", and a switch 65 is turned OFF, thereby preventing the reception voice from being output from a speaker 66. The output of the comparator 62 is inverted by an inverter 67, and a switch 68 is maintained in the ON position. Thus, either the switch 65 or 68 is turned OFF in accordance with the output of the comparator 62, thereby realizing a hands-free function. Reference numerals 69 and 70 indicate amplifiers. When the switch 65 is in the ON position and the switch 68 is in the OFF position, these amplifiers amplify the transmission voice signal to output a voice from the speaker 66.
By virtue of this hands-free function, a permanent one-way telephonic conversation is realized, in which the speaker sound is prevented from being picked up by the microphone and amplified. Further, in the case where the terminal on the other end of the line is an analog terminal or where an analog main wire (or analog station line) is used, it is possible to prevent the generation of howling caused by the returning transmission signal.
However, to ensure a hands-free function for such a telephone exchange system, it is necessary to use extension telephones having built-in hands-free circuits (echo suppressor circuits). If the hands-free function is to be ensured all over on the extension side, all the extension telephones must be hands-free types, resulting in a rather expensive system.
Further, in conventional private branch exchanges, an outside-line transfer may involve a reflection of a transmission signal as a result of a mismatching of the balancing circuit due to the two-wire/four-wire switching. Thus, adjustment of the voice-volume level will cause the generation of a howling noise.